I can't really help you with any of your questions, but I'm currently trying to produce Siamese Dumbos. I lucked out with a Craigslist Dumbo that happened to carry Siamese, and I've only just bred 2 of his Himalayan daughters back to him to see what I get. They're due the week after next.
I hadn't come across the info about breeding them with full black selfs, but luckily I'm already working on getting that color as well. I did read something about to get better blacks you should breed them with a blue, so I have a blue top-eared male that I saved from a litter that will replace one of my black hoods.
I'll keep my fingers crossed for ya!! Man, I really want some dwarves now. But I just got my first pair of double rexes, so I guess they'll have to hold me over lol.